The invention relates to pressure sensors adapted to measure the pressure of a fluid. More particularly, the invention relates to a pressure sensor having a capacitance type pressure capsule and a housing for supporting the pressure capsule.
The incorporation of pressure sensors into the electronic control systems of automotive vehicles poses severe operational requirements for the pressure sensor. These requirements are further heightened when the pressure sensor is used to measure the manifold pressure (MP) or the absolute manifold pressure (MAP) of the engine.
The pressure sensor must operate in a mobile and severely hostile environment which may include any of the following characteristics: varied temperature extremes, excessive shock and vibration and high levels of electromagnetic interference and corrosive gases and liquids. The pressure proximate the intake manifold is rapidly changeable and susceptable to large variations in magnitude (1-4 atmospheres) which may be caused as the result of explosive backfire or may occur during the boost phase of the operation of a supercharger or turbocharger; thus requiring a pressure sensor having a large dynamic range and high sensitivity. Irrespective of the above, if the present invention is utilized in an automobile, the requirements of the industry dictate that it must be (1) inexpensive, (2) repeatable, and (3) capable of being mass produced which implicitly requires the use of novel and rapid cost effective fabrication techniques as opposed to the slower ion-milling vacuum deposition methods such as sputter-etching techniques or brazing techniques as mentioned by Polye in U.S. Pat. No. 3,858,097 and by Dias, et al in U.S. Pat. No. 4,064,550.
The present invention is a pressure sensor of the capacitive type comprising a metallic housing to support a dual diaphragm capacitive pressure capsule. The pressure capsule comprises a pair of flat flexible fused quartz plates which are separated by a ring of dielectric material (such as a glass frit) defining an interior chamber which is maintained at a determinable pressure (vacuum) reference level. The pressure capsule contains a plurality of electrodes located within the interior chamber forming the conducting plates of a reference capacitor C.sub.r and pressure sensing capacitor C.sub.p. In particular, one plate, the upper plate contains a ground electrode while the other plate, i.e. the lower plate contains both the C.sub.p and C.sub.r electrodes. The lower plate may also contain an electrical shield on an external surface opposite the interior chamber. In addition, the pressure capsule contains a plurality of electrical contacts, one associated with each of the above electrodes. These contacts are located outside of the internal chamber near the edges of each of the flat plates. Furthermore, each flat plate contains a cutout oppositely situated relative to the electrical contacts on the other plate therein exposing each electrical contact for convenient access.
In response to an applied pressure, both plates act as cantilevered plates, and deflect towards each other and bend over the raised dielectric ring varying the capacitance between the plates.
The housing includes means for circumferentially sealing and compressively supporting both plates wherein the compressive forces on the plates are opposingly directed through the dielectric ring. The housing further includes means, such as a port, for communicating the pressure to be sensed to the upper plate proximate the general location of the interior chamber and transfer port means located therein to further communicate the pressure to the second or lower plate.
An advantage of the present invention is that the dual diaphragm or double plate deflection provides for a capacitor having higher sensitivity than one using a single deformable member. This feature permits the use of smaller electrodes to achieve the same change in capacitive output relative the change in applied pressure. An additional feature is the use of flat quartz plates which uniformly deform about the dielectric ring enchancing the linearity of the output signal and further eliminates the hollowed or etched cavities in deformable members shown in the prior art.
A further advantage is accomplished by the circumferential seal and support provided by the housing and o-rings which virtually floats the pressure capsule within the pressure environment to be measured. By using a resilient seal and support the pressure capsule is effectively isolated from shock and vibration.
A further advantage of the above-described seal and support is achieved because the compressive support forces are applied through the dielectric ring therein eliminating end loading which causes excess stress, non-linearity and premature failure. In addition, by supporting the pressure capsule as described, the electrical contacts are isolated from the hostile elements in the gas whose pressure is to be measured.
A further feature of the present invention is achieved by the orientation of the metallic housing to the capsule electrical components effectively shielding the pressure capsule from stray interference signals as well as preventing electrical signals from radiating from the housing and pressure capsule.
It is therefore an object of the present invention to provide a pressure responsive capacitive pressure capsule protectively supported within a coacting housing. These and other objects, features and advantages of the invention will be clear from the detailed description of the drawings.